Persons who are deaf or hearing impaired who cannot hear well enough to use the telephone commonly make use of communication terminals specifically constructed in design to enable such persons to converse over telephone lines. Such devices are referred to as telecommunication devices for the deaf, or TDD's, and include both a keyboard and a display connected to the telephone through a modem (modulator/demodulator). The modem is typically built into a TDD connected either by hard wiring directly to a telephone line or through an acoustic coupler which couples the modem to a normal telephone handset. TDD's are normally capable of transmitting information over a telephone line by means of coded tones to another compatible TDD connected at the opposite end of the telephone line through another modem.
There are several protocols that are used for transmitting digital information through analog lines such as telephone lines. The most commonly used information protocol in the electronics industry is referred to as ASCII (American Standard Code for Information Interchange). The ASCII code was designed for and is most commonly used for information interchange between computers. However, largely due to historic reasons, TDD's have operated on a different protocol, originally developed specifically for TDD communication. This TDD protocol is referred to here as the Baudot/Weitbrecht, or standard Baudot, and includes both a specific 5-bit Baudot code and a frequency shift keying (FSK) protocol of electronic communication. The standard Baudot communication is simplex, that is to say it is capable of only transmitting in one direction at one time. Therefore, during normal TDD communication, one station must be silent while the other is transmitting. It has become a convention that one TDD user informs the other TDD user when it is the other user's turn to utilize the communication link.
The inability of the traditional Baudot/TDD communication network to permit bi-directional or duplex communication of the network created by this form of communications has been an inadequacy since its inception. Under current TDD/Baudot communication protocols, if both users attempt to transmit at the same time, each station will only display to the user the characters it is transmitting. This is because standard Baudot TDD's are designed to give priority to transmission. Since prior TDD's cannot receive data while transmitting, when transmitting standard TDD's make no attempt to receive incoming characters. This creates obvious difficulties in the use of TDD systems for communication between individuals and makes such communication not similar to normal human communication. As in even a brief monitoring of oral communication between hearing individuals will indicate, human speech is characterized by constant interruptions and interchange. The current TDD/Baudot communication network is incapable of handling such interruptions and interjections, and hence is less similar to audible human conversations than would be desired in an ideal system.
In addition, it is often desired that during a TDD communication that one user be able to signal or interrupt the other user. Often, for example, one user may be launched onto a long description, or explanation, which the other user is already aware of or has heard before. In normal human audible conversation, a listener can indicate to the speaker that he has already heard that part of the explanation. In TDD communication, due to the simplex nature of the communication link, the receiver is unable to communicate with the transmitter until the transmission is complete. Since communications can sometimes be quite long, this is a source of frustration and time delay, and hampers normal human tendencies during conversation. Heretofore the standard Baudot/Weitbrecht network has been incapable of handling such tendencies.
There was one instance known of an attempt to permit interruption in a communication device operating under Baudot protocol. One of the early originators of TDD communications in the United States, Mr. Weltbrecht instituted a "news service" for the deaf community in the United States accessible by telephone. Mr. Weitbrecht constructed what was, in essence a recording device, which played out a periodic news compilation in Baudot communication to any TDD that would dial the phone number associated with the news service. In that time period, it was common for TDD's to listen for space tones (1800 Hertz) only, and to not even sense mark tones (1400 Hertz). A receiving TDD would simply assume absence of space tones during a character meant mark. Utilizing this characteristic, Weitbrecht constructed the news service device so that if it was to transmit a bit sequence of three marks (or 1's) at any point during the transmission, the device would simply stop transmitting the mark tone and listen for tones from the communicating station. If tones were sensed during the interval, the news service device would cease transmission. This feature was transparent to users at that time since most TDD's of the era did not detect mark tones. This characteristic is no longer true of modern electronic TDD's.